Glucoamylase structural, functional, and evolutionary relationships.

To correlate structural features with glucoamylase properties, a structure-based multisequence alignment was constructed using information from catalytic and starch-binding domain models. The catalytic domain is composed of three hydrophobic folding units, the most labile and least hydrophobic of them being missing in the most stable glucoamylase. The role of O-glycosylation in stabilizing the most hydrophobic folding unit, the only one where thermostabilizing mutations with unchanged activity have been made, is described. Differences in both length and composition of interhelical loops are correlated with stability and selectivity characteristics. Two new glucoamylase subfamilies are defined by using homology criteria. Protein parsimony analysis suggests an ancient bacterial origin for the glucoamylase gene. Increases in length of the belt surrounding the active site, degree of O-glycosylation, and length of the linker probably correspond to evolutionary steps that increase stability and secretion levels of Aspergillus-related glucoamylases.